The present invention relates to a digital speech network and more particularly, to a speech digitizer for digitizing an analog speech waveform for transmission over a serial digital channel in a digital communication system.
In the prior art, digital speech networks accept an accoustic speech signal and convert or translate it into a serial digital data stream. Originally, such devices tended to be bulky, costly and unreliable. Progress in the development of speech algorithms, plus the advances in digital technology and digital signal processing techniques, have reduced size and cost and increased reliability to a point where beneficial widespread use of such devices can be confidently predicted.
Generally, a digital speech network comprises an analyzer which converts the audio signal into a digital format which can then be transmitted over a conventional digital telephone channel and a synthesizer which is responsive to the digital information in order to reconstruct the audio signal.
Problems occuring in the prior art are (1) correctly estimating the excitation parameters in speech analysis-synthesis systems in which it must be determined whether an excitation signal is voiced or voiceless (periodic or random) and (2) estimating the time varying voice fundamental frequency (pitch). Speech quality is critically dependent upon the successful estimation of these two parameters-voice and pitch.
If an analyzer incorrectly identifies a voiceless sound to be voice, the listener hears an unpleasant "buzziness" in the synthesized speech. If the analyzer incorrectly identifies a voice sound (or part of a voice sound) to be voiceless, the sound suddenly becomes harsh. Mistakes in estimating fundamental frequency of the voice cause comparable high intrusive unnatural sounds to appear to be incorporated into the perceived speech. These effects can be noticeable even when the analyzer is correct for a large percent of the time. In difficult environments in which the analyzer causes a large percentage of mistakes, the effect is to severely lower the overall intelligibility and quality of the speech communications.
Therefore, in view of the above background, it is an objective of the present invention to provide a speech digitizer having improved pitch detection and voicing detection capabilities.